Yep. Will the same occur when copies of watermarked books appear though? It's become increasingly common for people to receive letters with flimsy evidence saying pay X or we take you to court (for downloaders based on IP), the same could apply to the source of any book, will anyone want to risk the cost of going to court to show they didn't upload it?

That is one of the issues at hand with the IP based copyright infringement suits. A lot of requests for discovery to find out who is associated with the IP address for a given time are thrown out, and some cases even after discovery are thrown out, because the account holder isn't liable for the infringement. Typically the only ones that stick are the ones where the person admits doing it.